In a LTE wireless communication system, Orthogonal Frequency Division Multiplexing (OFDM), as a core physical layer technology, modulates data stream on a plurality of orthogonal subcarriers, among which the frequency spectra can be overlapped with each other. As a result, the spectral efficiency will be increased remarkably.
Referring to FIG. 1, in an OFDM system, the transmitted information may be described with a resource grid, through which the entire time-domain and frequency-domain resources will be shown in the form of Resource Elements (RE). In other words, each smallest square in FIG. 1 corresponds to a time length of a subcarrier on the frequency domain and a symbol on the time domain, and the information to be transmitted is carried by the REs. The multiple REs constitute a resource block. To be specific, in the case of a normal cyclic prefix (CP), one resource block is formed by 12 subcarriers*7 symbols, and in the case of an extended CP, one resource block is formed by 12 subcarriers*6 symbols.
In order to demodulate the data correctly, it is necessary to know the channel condition at each RE position. In a LTE system, channel estimation at each RE position is obtained by inserting a known reference signal (RS) at a specific RE position for each resource block to calculate the channel estimation at the RE position, and then obtaining the channel estimation at all of the other RE positions by interpolation.
A channel estimation algorithm is usually performed in two steps. At first, frequency-domain channel estimation is performed in the frequency domain, i.e., with respect to a row of symbols that have RSs in FIG. 1, interpolation filtering is performed on initial channel estimation values of the RSs to obtain the frequency channel estimation for each symbol on the row, and then interpolation filtering is performed in the time domain in accordance with the result to complete time-domain channel estimation, i.e., to obtain the final channel estimation value. For clarification hereinafter, Hf(RS) is used to represent the frequency-domain channel estimation of the symbol carrying RS information, including the channel estimation obtained by frequency-domain interpolation.
The LTE system includes two modes, i.e., TDD and FDD. For the FDD mode, a UE may receive continuous downlink subframes, and for the TDD mode, the received downlink subframes are discontinuous, i.e., the RS distribution is discontinuous as shown in FIG. 2, in which D represents the downlink subframes and U represents uplink subframes.
Actually, even if the continuous downlink subframes can be received in the FDD mode, the RS distribution in one subframe is uneven in a normal CP mode. As shown in FIG. 3, the interval between the symbols carrying the RS information may include 3 or 4 symbols.
The key for the frequency-domain interpolation filtering and time-domain interpolation filtering is to select suitable filter structures and filter coefficients. However, due to the discontinuous and uneven RS distribution in the LTE system, it becomes a challenge to select the time-domain channel estimation filter and the coefficients thereof.